Samoilenko, A. M., Slyusarchuk, V. E., & Slyusarchuk, V. V. (1997). Investigation of a nonlinear difference equation in a Banach space in a neighborhood of a quasiperiodic solution. Ukrains’kyi Matematychnyi Zhurnal, 49(12), 1661–1676. https://umj.imath.kiev.ua/index.php/umj/article/view/5171